1. Field of the Invention
The present invention relates to a method for manufacturing of a magnetic circuit for use in a magnet-aided actuator for converting electric energy to kinetic energy etc.
2. Description of the Prior Art
For various kinds of office automation equipment, linear motors (voice coil motors), controllers, etc., for example, actuators of a variety of types have been developed. Concrete examples include an actuator having a magnetic coil and a magnetic core made movable and an actuator having a permanent magnet movable. In either case, the actuator comprises a yoke, a magnetic coil and a permanent magnet in combination.
Actuators using a permanent magnet have encountered problems as to how control precision is to be made high and how impellent is to be enhanced. Attempts have been made to solve the problems through modifications of the structures of the actuators. As a high-precision controllable actuator, for example, there has been proposed a linear actuator comprising an outer yoke and an inner yoke each having a magnetic coil wound around it and a cylindrical permanent magnet magnetized in the radial direction (refer, for example, to JP-A HEI 6-284670). A linear actuator comprising a side yoke, a first permanent magnet of a given polarity and a second permanent magnet of the opposite polarity, has been proposed as an actuator capable of enhancing the impellent (refer, for example, to JP-A HEI 5-49226).
The former prior art adopts the configuration in which the pair of outer yoke and inner yoke each having a magnetic drive coil would around it are coaxially disposed using a spacer, the cylindrical permanent magnet supported on a magnet holder and magnetized in the radial direction is disposed as a movable member between the magnetic drive coils, and an output shaft piecing through the axis portion of the inner yoke is connected to the magnet holder. According to this prior art, this configuration makes it possible to provide a linear actuator exhibiting highly precise controllability and enabling miniaturization and lightweight and higher versatility.
In the latter prior art, a hollow cylindrical permanent magnet is affixed onto the inner circumferential surface of the side yoke formed in a bottomed hollow cylindrical shape from a ferromagnetic material and, at the same time, a cylindrical center yoke made from a ferromagnetic material is disposed coaxially with the side yoke on the bottom of the side yoke in a projecting manner, thereby configuring a magnetic circuit. This prior art has a structure in which an axially movable member is disposed in a magnetic void defined between the permanent magnet and the center yoke. The permanent magnet comprises the first permanent magnet extending from the bottom of the side yoke to the neighborhood of an opening and the second permanent magnet magnetized to have a polarity opposite to that of the first permanent magnet and provided in the vicinity of the opening. By providing a drive coil and a detection coil, with the movable member facing the first and second permanent magnets respectively, it is made possible to materialize a linear actuator small in size, light in weight, enhanced in impellent and high in linearity and reliability.
While each of the former and latter prior art references uses a cylindrical permanent magnet in a magnetic circuit constituting an actuator, a rare earth sintered magnet exhibiting high magnetic intensity is formed into a cylindrical permanent magnet only with great difficulty from the manufacturing point of view.
Generally, a rare earth sintered magnet is produced by the powder metallurgy method comprising the steps of compact-molding raw alloy material in a magnetic field into a cylindrical shape and subjecting the compacted body to sintering and aging treatments. In this case, the compacted body is liable to be shrunk and deformed during the process of the sintering treatment. When such shrinkage arises, circularity is lowered to require polishing for adjustment of the outside and inside diameters before incorporating the magnet into the actuator, thus inducing an increase in man-hour and manufacturing cost. When the shape of the magnet is made cylindrical, since the volume relative to the weight becomes large, the number of the magnets to be treated at a time will be decreased to increase the sintering cost.
Though a difference is made depending on the structure of a magnetic circuit, where the dimensional accuracy in outside and inside diameters of a cylindrical rare earth sintered magnet is low when it is intended to affix the magnet to a yoke, the area of the magnet in contact with the yoke becomes substantially very small. As a result, the force of affixing the magnet to the yoke is considerably diminished to possibly necessitate concomitant use of affixation by an adhesive. Furthermore, a problem will arise when magnetizing the magnet. Specifically, a magnetizing apparatus large in size has to be used for the cylindrical magnet when magnetizing the magnet either before or after the magnet is affixed to the yoke.
Also in addition thereto, a cylindrical permanent magnet used in a magnetic circuit of an actuator generally has an orientation in the direction of diameter (a so-called radial orientation). A cylindrical rare earth sintered magnet having the radial orientation, when being formed in a small wall thickness, for example, sustains cracks or chips during the process of sintering, resulting in tendency to greatly lower the yield. This decrease in yield constitutes a serious cause of preventing an actuator from having high performance and from being miniaturized.
In view of the conventional state of affairs described above, the present invention has been proposed. An object of the present invention is to provide a magnetic circuit capable of materializing its high-performance property and miniaturization property without encountering any difficulty during the course of the manufacture thereof even when a rare earth sintered magnet is used as a permanent magnet. Another object of the present invention is to provide a method and apparatus for efficiently manufacturing the magnetic circuit. Still another object of the present invention is to provide an actuator and a speaker each having the magnetic circuit incorporated therein to attain its high-performance property.